112 IMBIBITION 



amount of water imbibed, in terms of the air-dry weight of the seeds, of nearly 

 15 per cent. Near the upper end of this range of concentrations an increase 

 in osmotic pressure of several hundred atmospheres is required to produce 

 an equivalent change in the volume of water imbibed. This is further evi- 

 dence that the first increments of water passing into any imbibant are held 

 by tremendously greater forces than those which are imbibed subsequently. 



3. Ionic Effects, — The amount of water which will be imbibed from a 

 given solution may also be influenced by the species of ions in that solution. 

 If agar or kelp stipe, for example, is allowed to come to equilibrium with 

 equimolar solutions of lithium, sodium, ammonium, and potassium chlorides 

 swelling will not be equal in the different solutions. The greatest imbibition 

 will occur in lithium chloride solution, next greatest in sodium chloride, next 

 greatest in ammonium chloride, and least in the potassium chloride solution. 

 In all of the solutions swelling will be less than in pure water (Fig. 23). 

 At first sight it would seem that this might be ascribed to an osmotic effect. 

 The potassium chloride solution might be assumed to have a higher osmotic 

 pressure than the lithium chloride solution, due to differences in dissociation or 

 hydration, and hence to exert more of a retarding effect upon imbibition than 

 the other solutions. Actually the opposite condition holds, hence this influence 

 of electrolytes upon imbibition cannot be explained in terms of an osmotic 

 effect. Apparently, since all of these electrolytes give rise to the same anion 

 the cause of this phenomenon must be looked for in terms of specific effects 

 of the cations upon swelling. 



Many other processes and physical properties are affected by this and 

 similar ionic sequences. Such ionic series as these are known as lyotropic 

 series, and such effects of ions as are found to be arranged in sequences of 

 this sort are called lyotropic effects. 



Two lyotropic series of cations, and one of anions, are generally recog- 

 nized, as follows : 



Li— Na— NH4— K— Rb— Cs 



Mg— Ca— Sr— Ba 



CNS— I— NO3— Br— CI— SO4 



Sometimes the order of the effect of a lyotropic series is from left to 

 right and sometimes the reverse depending upon the property affected. As 

 arranged above the series of univalent cations represents (reading from left 

 to right) the order of their increasing effectiveness in checking the imbibition 

 of water by kelp stipe. The same is true of the series of bivalent cations. 

 The anions in the third series do not differ very greatly in their effects upon 



